Looking backward to the attacks of five years ago and forward to November’s election, President Bush is appealing in dire yet optimistic terms to the American public for support and unity in the war.

“This struggle has been called a clash of civilizations. In truth, it is a struggle for civilization,” Mr. Bush said in a 16-minute prime-time televised address from the Oval Office. “We are fighting to maintain the way of life enjoyed by free nations. And we are fighting for the possibility that good and decent people across the Middle East can raise up societies based on freedom, and tolerance, and personal dignity.”

Five years after Islamic radicals hijacked four commercial airliners on a suicide mission that obliterated the World Trade Center and slashed a hole in the Pentagon, Mr. Bush spoke with a new urgency about the nation’s war on terror, warning that its outcome could shape the fate of generations of Americans to come.

“If we do not defeat these enemies now, we will leave our children to face a Middle East overrun by terrorist states and radical dictators armed with nuclear weapons,” Mr. Bush said. “We are in a war that will set the course for this new century — and determine the destiny of millions across the world.”

In his speech, the president in broad strokes sought to link the nation’s war in Iraq and the broader struggle against global radical Islamic movements. Defending the decision to topple Saddam Hussein, Mr. Bush said his administration, Congress, and the United Nations understood that Iraq was a threat. After the September 11, 2001, attacks, “Saddam’s regime posed a risk that the world could not afford to take.”

Al Qaeda is now using Iraq as a battlefront “to stop the rise of a free society,” Mr. Bush said, warning that the terrorist group would be “emboldened” by an early exit of American troops.

It is that linkage that Democrats have tried to break as they seek to capitalize on the unpopularity of the Iraq war among Americans as the November elections draw near.

Mr. Bush suggested that partisan criticism was undermining America’s efforts, as he urged the nation to “put aside our differences, and work together to meet the test that history has given us.”

“Winning this war will require the determined efforts of a unified country,” Mr. Bush said. “Every American wishes it were over. So do I. But the war is not over — and it will not be over until either we or the extremists emerge victorious.”

His speech was the capstone of a busy day of ceremonies for the president that started in the Lower East Side, where Mr. Bush met with firefighters and police officers at a firehouse. He then laid a wreath at a memorial site in Shanksville, Pa., which commemorates the victims of United Airlines Flight 93, whose passengers and crew revolted against the hijackers, sending the plane crashing into a barren field instead of its intended target, which was thought to be the White House or the Capitol building.

In Washington, D.C., Vice President Cheney attended a memorial service at St. John’s Episcopal Church, bringing along a surprise a guest, Margaret Thatcher, a former British Prime Minister. It was a rare public appearance for Ms. Thatcher, 80, who has suffered minor strokes.

In New York, thousands of friends and family members of the 2,749 who perished in the World Trade Center attack converged at ground zero to take part in the rituals of remembrance that have guided them through their grief.

“Five years from the date of the attack that changed our world, we’ve come back to remember the valor of those we lost — those who innocently went to work that day and the brave souls who went in after them,” Mayor Giuliani, standing on a platform facing clusters of mourners, said. Hundreds of relatives congregated above the ground zero to hear the slow and steady recitation of the names of victims, which has been the centerpiece of the ceremony.This year, the names were read by wives, husbands, boyfriends, exhusbands, ex-wives, girlfriends, and partners of the victims. In pairs, they came to the wooden podiums overlooking the 16-acre pit “not in the first flush of despair, but with the saving grace of memory,” as Mayor Bloomberg put it in his brief remarks opening the ceremony.

Mindful of the expanding gulf of time separating mourners from the mourned, many of the spouses who came to the microphone made vows to remember. “On this dark day, the greatest gift I could give you is the gift of remembrance,” said Joanne Grzelak, the wife of Joseph Grzelak, a chief of Battalion 48 who was among the 343 firefighters who died in the rubble.

“Slowly but surely, it’s getting out of people’s minds. You have to remember what exactly happened,” said Gary Roy, the brother of Timothy Roy, a police officer who was off duty on September 11 but who gave his life in the rescue.

For the first time, Mendel Ilowitz, a resident of Borough Park in Brooklyn whose brother Abraham Ilowitz was a broker for Cantor Fitzgerald in the North Tower, attended the ground zero ceremony. In past years, he said, “I couldn’t come. I couldn’t face it. I knew he was inside the rubble.”

Carrying red, orange, white roses, the families descended down a seven-story ramp to ground zero to place the flowers in the two reflecting pools designating the footprints of the towers.

As bells tolled throughout the city, they bowed their heads during the four moments of silence that represented a timeline of the terror: at 8:46 a.m. when American Airlines Flight 11 crashed into North Tower; at 9:03 a.m. when United Airlines Flight 175 engulfed the South Tower in flames, at: 9:59 a.m. when the South Tower collapsed, and at 10:29 a.m. when the North Tower fell, completing the catastrophe.

Outside ground zero, Mr. Giuliani, who is considering a run for the White House, faced questions from reporters about charges made by a former EPA administrator, Christine Todd Whitman, that the city in the aftermath of the attacks failed to force ground zero recovery workers to wear respirators that would have helped to protect them.

Mr. Giuliani said the city “certainly gave people instructions that they could wear masks,” and said he doesn’t recall any warnings coming from Ms. Whitman about the dangers. “What I remember from Christine Whitman is her saying the air was fine and her saying that quite prominently over and over again,” he said. Mr. Giuliani said he and his top aides spent hundreds of hours at ground zero in the months following the attack and also left themselves vulnerable to respiratory problems.